Apparatus and method for distributing beads

ABSTRACT

Substrate beads for combinatorial synthesis are selected individually from a mixture by suspending the mixture in an electrically conductive liquid, in a bead selection vessel, causing the liquid to flow at a controlled rate through an aperture in the side wall of a pipette extending through the vessel, and detecting the passage of a bead through the aperture by monitoring an electrical resistance across the aperture. In an alternative embodiment, beads are passed through a tube into a collection passage in which a continuous laminar flow takes place. Detection takes place at the tip of the tube, and, in response to the detection of a bead, the flow through the collection passage is diverted to cause the bead to be deposited. In both cases, the selected bead is deposited into a well of a plate having rows and columns of wells in a rectangular array, while a vacuum is drawn through a filter in the bottom of the well. The bead selection head is moved from well to well in each column, and the well plate is indexed to position the columns successively underneath the path of the bead selection vessel. A plate handling mechanism retrieves plates from a supply stack, moves them laterally underneath the bead selection vessel, and elevates them into another stack.

This application is a 371 of PCT/GB97/02883 filed Oct. 17, 1997 which isa continuation of Ser. No. 08/734,228 filed Oct. 21, 1996 Abandon.

FIELD OF THE INVENTION

This invention relates to chemical synthesis, and more particularly toan improved apparatus and method for distributing microscopic beads ofthe kind used as substrates in combinatorial chemistry.

BACKGROUND OF THE INVENTION

In combinatorial synthesis, it is often desirable to be able todistribute beads into a two-dimensional array, so that each variant in acombinatorial library can be identified by its position in the array.The array can consist of a set of plates, each having rows and columnsof wells, with one bead, or some other predetermined number of beads, ineach well. The beads are typically made of polystyrene, and serve assubstrates for different compounds produced in the process of split andcombine synthesis. Ultimately, the synthesized compounds are strippedfrom the beads and tested for activity. The identity of an activecompound is determined by spectrographic analysis, in the light of theinformation available concerning the reaction histories of the beadsbeing distributed.

The beads are spherical and of extremely small size, e.g. 300 mm indiameter. Consequently, they are difficult to handle, and it has beenvery difficult to separate a single bead from a mixture of beads.

SUMMARY OF THE INVENTION

The principle object of this invention is to provide an apparatus andmethod for selecting individual beads, or preselected numbers of beadsfrom a mixture of beads, and distributing the selected beads into atwo-dimensional array.

A further object of the invention is to provide a bead distributionapparatus which is both simple and highly reliable.

This apparatus utilizes a head similar to that of a “Coulter” counter, adevice used to count and size particles in a liquid. For example, it isused in the petroleum industry to assess engine wear by countingparticles in lubricating oil. The principle on which the Coulter counteroperates is that electrical resistance of a conductive fluid, measuredby electrodes on both sides of a small aperture, increases momentarilyas a solid particle passes through the aperture. The passage ofparticles through the aperture is detected as a electrical pulses, whichcan be counted electronically. The Coulter counter is described indetail in U.S. Pat. No. 2,656,508, issued Oct. 20, 1953, and thedisclosure of that patent is here incorporated by reference.

The preferred embodiment of this invention takes advantage of theprinciple of the Coulter counter, but uses the principle in a differentway and for a different purpose.

In accordance with the invention, beads, from a mixture of beads,preferably of substantially uniform size, are distributed into an arrayhaving multiple locations, so that a predetermined number of beads isdeposited at each location in the array. This is carried out by forminga suspension of the mixture of beads in a carrier liquid; causing a partof the liquid to flow through an aperture of a size such that the beadscan pass through the aperture only one at a time; detecting the passageof a predetermined number of the beads through the aperture; and, inresponse to the detection of the passage of the predetermined number ofthe beads through the aperture, depositing them at a predeterminedlocation of the array.

In one embodiment of the invention, the selection of beads to bedeposited is carried out by discontinuing the flow through the apertureupon detection of the passage of the predetermined number of the beadsthrough the aperture. In an alternative embodiment, flow takes placecontinuously through the aperture, and is diverted in response to adetection signal to effect bead deposition.

The carrier liquid is electrically conductive, and is stirred to keepthe beads in suspension. In a first embodiment, to deposit a singlebead, a syringe is operated to produce a steady flow of liquid throughan aperture in the side wall of a tube extending through the containerfor the carrier liquid. Eventually, a bead will pass through theaperture along with the liquid. When the passage of a bead is detectedelectrically, the operation of the syringe is discontinued and the flowof liquid through the aperture stops. This prevents other beads frompassing through the aperture. After its passage through the aperture isdetected, the bead is flushed out of the tube by a pumped liquid, anddeposited at its location in the array, preferably into a well in a wellplate. Preferably, while the syringe is causing liquid to flow into thetube through the aperture, liquid is withdrawn from the upper end of thetube by a pump at the same rate at which it flows into the tube throughthe aperture. This prevents liquid from passing through the lower end ofthe tube. Normally only one bead will be deposited at each location inthe array. However, multiple beads can be deposited at each location.This is done by counting the electrical pulses corresponding to peaks inresistance. When the desired number of beads is counted, the flow of thesuspension liquid is discontinued.

In a second embodiment, the aperture at which detection takes place isat the end of a tube through which liquid flows continuously. When abead is detected at the aperture, a signal is produced causing the flowof liquid to be diverted so that the bead is carried to the location atwhich it is to be deposited.

In a preferred embodiment of the invention, a stack of empty well platesis initially placed in the apparatus. The lowermost well plate in thestack is automatically moved to a position underneath the head with afirst row of wells positioned underneath, and parallel to a linear pathof movement of the head. The head moves successively from one well tothe next, depositing a bead in each well of the column. The well platesare indexed laterally to position successive rows of wells underneaththe path of the head. When a plate is filled, i.e. it has one bead ineach of its wells, it is moved into a new stack and the apparatusretrieves a new plate from the supply stack and begins to distributebeads to the new plate.

Further objects, details and advantages of the invention will beapparent from the following detailed description, when read inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic front view of a bead distribution apparatus,showing a movable bead distribution head, and mechanisms fortransporting well plates from a supply stack to a location underneaththe distribution head, and from the distribution head to a second stack;

FIG. 2 is a diagrammatic top plan view of the bead distributionapparatus;

FIG. 3 is a vertical section through the bead distribution head;

FIG. 4 is a fragmentary sectional view, showing details of the beadselection aperture in the bead distribution head;

FIG. 5 is a top plan view of the cover of the bead distribution head;

FIG. 6 is a section taken on plane 5—5 in FIG. 4; and

FIG. 7 is a section taken on plane 6—6 in FIG. 4;

FIG. 8 is a schematic diagram showing the fluid paths and controls ofthe apparatus, and illustrating the manner in which a bead is selectedfrom a suspension of beads in a liquid;

FIG. 9 is a schematic diagram illustrating the movement of well platesfrom the supply stack to a location underneath the distribution head,and from the distribution head to the second stack;

FIG. 10 is a typical plot of electrical voltage versus time across theaperture of the bead distribution apparatus; and

FIG. 11 is a schematic diagram showing a distribution head assembly inaccordance with an alternative embodiment of the invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, the bead distribution apparatus 10 comprisesa loading guide 12 in which empty well plates, of a standard,commercially available type, may be stacked. The empty well plates areunloaded from the bottom of guide 12 by an unloader 14 into a conveyor18, and are transferred individually by the conveyor to a positionunderneath a distribution head 16. Each well plate, e.g. well plate 19in FIG. 2, has a rectangular array of wells disposed in rows andcolumns. While a well plate is underneath the distribution head, thedistribution head moves from well to well in a row (perpendicular to theplane of FIG. 1), depositing a bead in each well. When the distributionhead has traversed a row of wells, the conveyor 18 indexes the wellplate to position a next row underneath the path of the head. This isrepeated until beads are deposited in all of the wells. Then theconveyor moves the well plate to a position underneath a stacking guide20, and the well plate is loaded into the stacking guide by a stacker22.

The unloader 14 and the stacker 22 are elevators with platforms whichengage the lowermost well plates in the guides and move vertically toload and unload the conveyor. Catches (not shown in FIGS. 1 and 2) areprovided at the lower end of the loading guide 12 for supporting thestack of well plates in the loading guide when the unloader 14 isretracted. These catches are electrically operated, and microprocessorcontrolled so that they cooperate with the unloader 14, allowing theunloader to receive a well plate from the loading guide 12 and depositthe well plate onto the conveyor. Ratchet-type catches (not shown inFIGS. 1 and 2) are provided at the lower end of the stacking guide 20 tosupport well plates in the stacking guide while allowing the stacker 22to deposit the well plates therein.

While the well plate is underneath the distribution head 16, a bead isdeposited in each well, the distribution head being indexed from well towell in each row of wells along a supporting arm 24. The distributionhead can also be moved to a position just above a waste collector 25,which, as shown in FIG. 2, is located behind the path of movement of thewell plates.

The distribution head supporting arm 24 is itself movable vertically sothat the distribution head can be moved up and down. All of the fluidconducting lines and electrical leads to the distribution head 16 areflexible, and preferably bundled together in a single flexible sheath 15(FIG. 1), so the that the distribution head can move freely.

The distribution head, shown in detail in FIG. 3, comprises a cup-shapedvessel 26 having a cover 28 secured to a flange 30 of the vessel by athreaded ring 32. A sleeve 34 extends through the center of the cover,and a pipette 36 is held in the sleeve by seals 38 and 40. The pipetteextends through vessel 26, its upper end 42 being located above thecover 28, and its lower end 44 being below the bottom of the vessel. Thepipette extends through a seal 46 at the bottom of the vessel.

The outside wall of the pipette is cylindrical throughout most of itslength. However, a flat area 48 is formed on the outer wall of thepipette at a location within the interior of the vessel 26 just belowthe lower end of sleeve 34. As shown in FIG. 4, an opening 50 is formedin the wall of the pipette within the flat area 48, and a small watchjewel 52, having an accurately machined aperture 54 is secured to theflat area by an adhesive, with aperture 54 in register with opening 50.The aperture 54 is typically 500 mm in diameter, but can be larger orsmaller, depending on the size of the beads to be distributed. Ingeneral, the diameter of the aperture should be less than twice thediameter of the beads. Alternatively, the aperture can be formeddirectly in the wall of the pipette, obviating the use of the jewel.

As shown in FIG. 4, a first platinum electrode 56 is located within thepipette, with its tip adjacent to opening 50. Electrode 56 extendsthrough a seal 60 in the wall of the pipette and is connected to aflexible, multistrand lead 58, which is isolated from the suspension invessel 26 by sleeve 34. Another platinum electrode 62 has its tiplocated adjacent to the outer end of aperture 54, and extends throughseal 40 into the space between the pipette and the inner wall of sleeve34, where it is connected to a flexible lead 64.

Returning to FIG. 3, the top of the pipette is provided with a fitting66 for connection to a pump. The pump (not shown in FIG. 3) is used towithdraw liquid from the upper end of the pipette as it flows into thepipette through aperture 54. The pump is also used to deliver liquid forflushing selected beads out through the lower end of the pipette.

FIGS. 5, 6 and 7 show that the cover 28 has three additional openingsbesides its central opening through which sleeve 34 extends. The firstopening, 68, is an opening for introducing beads into the vessel, and isclosable by a removable threaded plug 70 (FIG. 3). The second opening,72, is an inlet for connection to a syringe used to produce flow ofliquid into vessel 26. The third opening, 74, is a vent opening. Asshown in FIGS. 3, 6 and 7, the underside of the cover 28 has an annulargroove 76 which varies in depth, with its deepest point being at thelocation of vent opening 74. The groove becomes continuously shallowerin both directions from the vent opening toward a shallowest point 78(FIG. 7) opposite the vent opening so that air can be exhaustedcompletely from the vessel. A porous filter 80 is provided in the ventopening.

FIG. 8 shows vessel 26 filled with a suspension consisting of a liquid82 and beads 84, held in suspension by an external magnetic stirringmechanism 86 cooperation with an agitator 88 inside the vessel. Thedensity of the beads 84 is preferably greater than that of the liquid,so that they sink in the liquid. To prevent surface tension from causingthe beads to float, they are pre-treated so that they are wetted by theliquid.

An example of an ideal liquid for use with polystyrene beads is asolution of ammonium acetate (2% w/w), and ammonium carbonate (2% w/w).

The liquid 82 is supplied to vessel 26 from a supply container 90through a peristaltic pump 92, which is controlled by a control unit 94,and a check valve 93. A syringe 96 is used to control the flow of theliquid after the vessel 26 is filled. The syringe is operated by anactuator 98, controlled by the control unit. When the plunger of thesyringe is withdrawn, the syringe draws liquid from supply container 90through a check valve 100. Forward movement of the plunger causes theliquid to flow through check valve 101 into vessel 26.

A reversible peristaltic pump 104 is provided to withdraw liquid fromthe upper end of the pipette 36 as it flows into the pipette throughaperture 54, and to pump liquid through the pipette from container 102for the purpose of flushing beads out of the pipette either to thewells, or to the waste collector 25. Pump 104 is also under the controlof unit 94.

In the operation of the system, the beads are introduced into the vessel26 through opening 6 8 (FIGS. 5 and 6), which is then closed by plug 70(FIG. 3). The control unit then operates pump 92 to fill the vessel withliquid from supply container 90. The operation of the pump 92 isdiscontinued when the vessel is filled with liquid. In the meanwhile, awell plate 106 is removed from the supply stack in guide 12 (FIG. 1) andmoved into position underneath the vessel 26, which is a part of thedistribution head.

The well plate 106, as shown in FIG. 1, comprises a two-dimensionalrectangular array of wells 108 (FIG. 8), each of which is preferablyclosed at its bottom.

With the distribution head positioned so that the pipette is over thewaste collector 25, the syringe 96 is operated to initiate a controlledflow of liquid at a constant rate into vessel 26 through check valve101. While other kinds of pumps can be used to carry out this operation,the syringe is desirable because it is capable of producing a steadyflow of liquid at a very slow rate. While the syringe 96 is causingliquid to flow into the vessel 26 and through the aperture 54 into thepipette 36, pump 104 is operated to withdraw liquid from the upper endof the pipette 36. The pump withdraws liquid at the same rate at whichit is being introduced into the pipette by the operation of the syringe96. The result is that liquid is prevented from being forced out the tipof the pipette, no matter how long it takes for a bead to pass throughthe aperture. This prevents unnecessary flow of liquid into the wastecollector, and also prevents loss of beads through the tip of thepipette when multiple beads are being collected in the pipette fordeposit into a well.

As seen in FIG. 8, a bead 110 in the suspension will eventually passthrough aperture 54 into the interior of the pipette. The passage of thebead is detected by the control unit 94 as a change in the resistancemeasured between platinum electrodes 56 and 62. When the control unitdetects the passage of a single bead, it causes actuator 98 to stoppushing the plunger of syringe 96 and simultaneously stops pump 104. Asa result, the liquid flow through the aperture is discontinued, and onlya single bead passes into the pipette. The control unit moves thedistribution head to the next well in sequence, and after apredetermined delay, during which the bead inside the pipette settles bygravity to the tip of the pipette, the control unit activates pump 104in the opposite direction for a short interval just sufficient to washthe bead out of the pipette 36 into the well.

The control unit then causes the distribution head to return to thewaste collection point, and the bead depositing operation is repeatedfor each well in the row. After beads are deposited in each well in arow, the well plate is indexed to position another row of wellsunderneath the path of the distribution head. The bead depositingoperation continues until beads are deposited in each well in the wellplate, whereupon, the well plate is transferred to a position underneaththe stacking guide 20, and elevated into the stacking guide. Themovement of the well plates is depicted in FIG. 9.

FIG. 9 also depicts laterally-slidable catches 111, which are operatedby actuators (not shown) under the control of the control unit, forsupporting the stack of well plates in the loading guide. Also shown arethe ratchet-type catches 113, which support well plates in the stackingguide while allowing well plates to be raised into the stacking guide 20by stacker 22.

The passage of beads through the aperture 54 in the pipette of thedistribution head is detected by measuring changes in the electricalresistance across the aperture between electrodes 56 and 62. Preferablythis is achieved by applying a current to the aperture by means of aconstant current source 126, as shown in FIG. 8, and monitoring thevoltage across the aperture. Alternating current is preferred in orderto avoid the effects of polarization. The passage of a bead through theaperture results in an increase in the resistance across the aperturemanifested by an increase in the voltage measured across the aperture.The voltage variation is depicted in FIG. 10, in which the voltage levelremains essentially constant except when a bead passes through theaperture, at which time a voltage peak 128 appears. The peak is detectedin the control unit and used to produce a signal to stop the operationof actuator 98 and pump 104.

As will be apparent from the description, the apparatus reliably selectsindividual beads from the mixture and deposits them in wells in the wellplates.

Although the flow of liquid through aperture 54 is stopped almostimmediately when the passage of a bead through the aperture is detected,occasionally, more than one bead will pass through the aperture into thepipette. The accidental passage of multiple beads through the aperturewill be detected by the control system as a series of voltage peaks, andthe control system responds by causing the distribution head to remainover the waste collector 25 while the beads are flushed out of thepipette. These beads can be reintroduced into the cup-shaped vessel 26for later distribution. Waste liquid is delivered to a closed container122, and a vacuum is drawn continuously on the waste collector 25,through container 122, by pump 120.

In the alternative embodiment shown in FIG. 11, a suspension of beads130 is established in a vessel 132. The vessel comprises a cylinder 134having a top closure 136 and a bottom closure 138. The top closure has abottom face 140 in the form of a symmetrical cone. A fluid inlet isprovided at 142, and an air outlet 144 at the peak of the cone has afilter 146. The bottom closure 138 has its top face 150 in the form ofan asymmetric cone with an emptying port 152 provided with a valve 154.

A metal tube 158 extends upward through the bottom closure 138 to alocation within the cone defined by bottom face 140 of the top closure.The tube is coaxial with that cone and has an opening 160 at its upperend for receiving beads along with liquid from the suspension 130. Thebeads are maintained in suspension by a magnetic flee 162 operated by anexternal magnetic stirrer 163.

The cone tends to concentrate beads at the location of the end opening160 of tube 158, and beads enter the tube 158 along with liquid.

The lower part of tube 158 extends through the bottom closure 138 of thevessel 132, and into an insert 164 of PTFE or other similar materialwhich is not electrically conductive. The insert fits into the upper endof a passage 166 in a metal block 168, and tube 158 extends into theinsert to a location near, but spaced from the lower end of the insert.The lower end of the insert has an opening 170, having a diameter equalto the internal diameter of the tube 158 so that the tube and opening170 form a continuous, smooth passage for the flow of liquid and beads.

The exterior of the lower end of insert 164 is narrower than the portionof passage 166 surrounding it, thereby providing an annular space 172for the flow of liquid received through a passage 174. The liquidintroduced through the passage 174 flows past the tip of insert 164,downwardly through a tapered part 176 of passage 166, and outwardlythrough an exit opening 178. The tip of the insert and the passage 166are gradually tapered and shaped so that the flow past the tip of theinsert is laminar.

The flow of liquid through passage 174 draws beads individually from thetip of insert 164 and delivers them into the upper part of passage 178,which serves as a collection chamber, from which they can be depositedin an array through opening 180. A solenoid-operated valve 182, locatedabove opening 180, is movable in a direction perpendicular to the planeof the drawing to permit or block flow through opening 180. This valveis shown in its closed condition. A branch 184, communicating withpassage 178, is provided with a similar solenoid-operated valve 186,which is shown in its opened condition. The valves 182 and 186 areprovided with hollow internal passages through which liquid can becaused to flow for washing the passages 178 and 184.

In the apparatus of FIG. 11, the tube 158 serves as one of twoelectrodes, and block 168 serves as the other electrode. Passage of abead through opening 170 effects a change in the electrical resistancemeasurable between tube 158 and block 168 in the same manner in which abead passing though aperture 54 in FIG. 4 affects the resistancemeasured between electrodes 56 and 62. In the absence of an electricalsignal produced by the passage of a bead through opening 170, valve 182is closed and valve 186 is open, allowing liquid entering the block 168through passage 174 to flow to waste through branch 184. The electricalsignal produced in response to the passage of a bead controls theoperation of valves 182 and 186 in such a way that valve 182 opensmomentarily and valve 186 closes. The time delay between the detectionsignal and the operation of the valves is set in relation to velocity ofmovement of beads in the space below the tip of tube 158, so that valve182 opens and valve 186 closes precisely at the time that the detectedbead is in close proximity to the connection of the branch 184 topassage 178. The operation of the valves allows beads to be depositedindividually into a suitable array, for example into wells in amicrotitre plate.

In the apparatus of FIG. 11, the fluid introduced through passage 174serves as a sheath fluid, and may be the same as the suspension fluidpassing downwardly through tube 158 from vessel 132. The sheath fluidflows in the same direction in which the beads move through tube 158.Preferably, the suspension fluid and the sheath fluid flow throughpassage 178 in laminar flow, i.e. in substantially non-mixing layersrespectively of the suspension fluid and the sheath fluid. Such laminarflow helps the beads to flow through passage 178 along a substantiallystraight path. the sheath fluid also helps to even out the flow of beadsand assists in achieving suitable serial separation of beads. The sheathflow may be controlled to optimize the flow of beads through passage178.

Various modifications can be made to the apparatus and processdescribed. For example, by incorporating an electronic counter in thecontrol unit, it is possible to count electrical peaks and disable thesyringe actuator only after a predetermined number of peaks is counted.In this way, if desired, a preselected number of beads can be depositedin each well. The control system can be programmed to cause the beads inthe pipette to be flushed into the waste collector if the number ofbeads passing through the aperture into the pipette exceeds thepreselected number.

The pipette (with its aperture 54) can be readily removed for cleaning,or for replacement by another pipette having a different aperture. It ispossible to eliminate the jewel 52 altogether, and thereby avoid thepotential problems resulting from detachment of the jewel from thepipette. This can be done by forming the aperture directly in the wallof the pipette, provided that the wall thickness is sufficiently small.

In still another modification of the apparatus, openings are provided atthe bottoms of the wells in the well plates, and filters are situated inthe openings. A vacuum head is situated underneath the path of thedistribution head and engageable with the undersides of the well plates.A vacuum is drawn continuously though the vacuum head, and is used toremove liquid form the wells. This keeps the wells from overflowing, andis an alternative to the previously described withdrawal of liquid fromthe upper end of the pipette at the same rate at which it enters thepipette through aperture 54.

In the embodiment of FIG. 11, various alternative valves and flowpassage configurations can be used, and fluidic control can be utilizedto divert the flow of liquid from one passage to another.

Still other modifications can be made to the apparatus and processwithout departing from the scope of the invention as defined in thefollowing claims.

What is claimed is:
 1. A process for distributing beads, from a mixtureof beads of uniform size, into an array having multiple locations, sothat a predetermined number of beads is deposited at each location inthe array, comprising the steps of: forming a suspension of the mixtureof beads in a liquid; causing at least a part of the liquid to flow, ina continuous stream, through a tubular passage alone with beads carriedby the liquid, one after another, through the tube and outwardly fromthe tubular passage through an aperture; causing liquid to flow from theaperture through a first path of a pair of paths by maintaining a valvein the other of said paths in a closed condition; detecting the passageof a predetermined number of the beads through the aperture; and inresponse to the detection of the passage of said predetermined number ofthe beads through the aperture, causing the liquid to flow through theother of said paths by opening the valve in said other path whileclosing a valve in the first path, thereby depositing said predeterminednumber of beads at a predetermined location of the array.
 2. The processaccording to claim 1 in which the liquid is electrically conductive, inwhich the tubular passage is defined in part by a conductive tube, andthe aperture is defined by an insulating element, and in which thedetecting step is carried out by monitoring the electrical resistance,across the aperture, between said conductive tube and an electrodeexternal to said tubular passage.
 3. The process according to claim 1 inwhich the liquid is electrically conductive, in which the tubularpassage is defined in part by a conductive tube, and the aperture isdefined by an insulating element, and in which the detecting step iscarried out by establishing a constant electric current in the liquidthrough the aperture, between said conductive tube and an electrodeexternal to said tubular passage, and monitoring the voltage across theaperture.
 4. Apparatus for distributing beads, from a mixture of beadsof uniform size, into an array having multiple locations, so that apredetermined number of beads is deposited at each location in thearray, comprising: means for containing a suspension comprising amixture of beads suspended in a liquid; means for collecting beads;means providing a tubular passage leading from said containing means tothe collecting means, said tubular passage having an outlet aperture;means for causing liquid to flow through the tubular passage, andoutwardly through the aperture, from the containing means to thecollecting means; means for detecting the passage of beads through theaperture; means establishing a pair of paths branching from theaperture, each path having a valve, the valve in one of said paths beingnormally closed and the valve in the other of said paths being normallyopen; and means, responsive to the detecting means, for opening thevalve in said one path and closing the valve in said other path, therebydepositing beads collected in the collecting means at predeterminedlocations in an array when the passage of a predetermined number ofbeads through the aperture is detected by the detecting means.
 5. Theapparatus according to claim 4, in which the tubular passage is definedin part by a conductive tube, and the aperture is defined by aninsulating element, and having an electrode external to said tubularpassage, and in which the means for detecting the passage of beadsthrough the aperture comprises means for monitoring electricalresistance across the aperture, between said conductive tube and saidelectrode external to said tubular passage.
 6. The apparatus accordingto claim 4, in which the tubular passage is defined in part by aconductive tube, and the aperture is defined by an insulating element,and having an electrode external to said tubular passage, and in whichthe means for detecting the passage of beads through the aperturecomprises means for establishing a constant electric current in theliquid through the aperture, between said conductive tube and saidelectrode external to said tubular passage, and means for monitoring thevoltage across the aperture.